[Code of Federal Regulations]
[Title 15, Volume 3]
[Revised as of January 1, 2004]
From the U.S. Government Printing Office via GPO Access
[CITE: 15CFR921.82]

[Page 110-117]
 
                  TITLE 15--COMMERCE AND FOREIGN TRADE
 
CHAPTER IX--NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION, DEPARTMENT 
                               OF COMMERCE
 
PART 921_NATIONAL ESTUARINE RESEARCH RESERVE SYSTEM REGULATIONS--Table of Contents
 
            Subpart I_General Financial Assistance Provisions
 
Sec. 921.82  Amendments to financial assistance awards.

    Actions requiring an amendment to the financial assistance award, 
such as a request for additional Federal funds, revisions of the 
approved project budget or original scope of work, or extension of the 
performance period must be submitted to NOAA on Standard Form 424 and 
approved in writing.

      Appendix I to Part 921-- Biogeographic Classification Scheme

                                 Acadian

    1. Northern of Maine (Eastport to the Sheepscot River.)
    2. Southern Gulf of Maine (Sheepscot River to Cape Cod.)

                                Virginian

    3. Southern New England (Cape Cod to Sandy Hook.)
    4. Middle Atlantic (Sandy Hook to Cape Hatteras.)
    5. Chesapeake Bay.

                               Carolinian

    6. North Carolinas (Cape Hatteras to Santee River.)
    7. South Atlantic (Santee River to St. John's River.)
    8. East Florida (St. John's River to Cape Canaveral.)

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                               West Indian

    9. Caribbean (Cape Canaveral to Ft. Jefferson and south.)
    10. West Florida (Ft. Jefferson to Cedar Key.)

                               Louisianian

    11. Panhandle Coast (Cedar Key to Mobile Bay.)
    12. Mississippi Delta (Mobile Bay to Galveston.)
    13. Western Gulf (Galveston to Mexican border.)

                               Californian

    14. Southern California (Mexican border to Point Conception.)
    15. Central California (Point Conception to Cape Mendocino.)
    16. San Francisco Bay.

                                Columbian

    17. Middle Pacific (Cape Mendocino to the Columbia River.)
    18. Washington Coast (Columbia River to Vancouver Island.)
    19. Puget Sound.

                               Great Lakes

    20. Lake Superior (including St. Mary's River.)
    21. Lakes Michigan and Huron (including Straits of Mackinac, St. 
Clair River, and Lake St. Clair.)
    22. Lake Erie (including Detroit River and Niagara Falls.)
    23. Lake Ontario (including St. Lawrence River.)

                                  Fjord

    24. Southern Alaska (Prince of Wales Island to Cook Inlet.)
    25. Aleutian Island (Cook Inlet Bristol Bay.)

                               Sub-Arctic

    26. Northern Alaska (Bristol Bay to Damarcation Point.)

                                 Insular

    27. Hawaiian Islands.
    28. Western Pacific Island.
    29. Eastern Pacific Island.

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[GRAPHIC] [TIFF OMITTED] TC12SE91.000

   Appendix II to Part 921-- Typology of National Estuarine Research 
                                Reserves

    This typology system reflects significant differences in estuarine 
characteristics that are not necessarily related to regional location. 
The purpose of this type of classification is to maximize ecosystem 
variety in the selection of national estuarine reserves. Priority will 
be given to important ecosystem types as yet unrepresented in the 
reserve system. It should be noted that any one site may represent 
several ecosystem types or physical characteristics.

                        Class I--Ecosystem Types

                           Group I--Shorelands

    A. Maritime Forest-Woodland. That have developed under the influence 
of salt spray. It can be found on coastal uplands or recent features 
such as barrier islands and beaches, and may be divided into the 
following biomes:

    1. Northern coniferous forest biome: This is an area of 
predominantly evergreens such as the sitka spruce (Picea), grand fir 
(Abies), and white cedar (Thuja), with poor development of the shrub and 
herb leyera, but high annual productivity and pronounced seasonal 
periodicity.

    2. Moist temperate (Mesothermal) coniferous forest biome: Found 
along the west coast of North America from California to Alaska, this 
area is dominated by conifers, has relatively small seasonal range, high 
humidity with rainfall ranging from 30 to 150 inches, and a well-
developed understory of vegetation with an abundance of mosses and other 
moisture-tolerant plants.

    3. Temperate deciduous forest biome: This biome is characterized by 
abundant, evenly distributed rainfall, moderate temperatures which 
exhibit a distinct seasonal pattern,

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well-developed soil biota and herb and shrub layers, and numerous plants 
which produce pulpy fruits and nuts. A distinct subdivision of this 
biome is the pine edible forest of the southeastern coastal plain, in 
which only a small portion of the area is occupied by climax vegetation, 
although it has large areas covered by edaphic climax pines.

    4. Broad-leaved evergreen subtropical forest biome: The main 
characteristic of this biome is high moisture with less pronounced 
differences between winter and summer. Examples are the hammocks of 
Florida and the live oak forests of the Gulf and South Atlantic coasts. 
Floral dominants include pines, magnolias, bays, hollies, wild tamarine, 
strangler fig, gumbo limbo, and palms.

    B. Coast shrublands. This is a transitional area between the coastal 
grasslands and woodlands and is characterized by woody species with 
multiple stems and a few centimeters to several meters above the ground 
developing under the influence of salt spray and occasional sand burial. 
This includes thickets, scrub, scrub savanna, heathlands, and coastal 
chaparral. There is a great variety of shrubland vegetation exhibiting 
regional specificity:

    1. Northern areas: Characterized by Hudsonia, various erinaceous 
species, and thickets of Myricu, prunus, and Rosa.
    2. Southeast areas: Floral dominants include Myrica, Baccharis, and 
Iles.
    3. Western areas: Adenostoma, arcotyphylos, and eucalyptus are the 
dominant floral species.
    C. Coastal grasslands. This area, which possesses sand dunes and 
coastal flats, has low rainfall (10 to 30 inches per year) and large 
amounts of humus in the soil. Ecological succession is slow, resulting 
in the presence of a number of seral stages of community development. 
Dominant vegetation includes mid-grasses (5 to 8 feet tall), such as 
Spartina, and trees such as willow (Salix sp.), cherry (Prunus sp.), and 
cottonwood (Pupulus deltoides.) This area is divided into four regions 
with the following typical strand vegetation:
    1. Arctic/Boreal: Elymus;
    2. Northeast/West: Ammophla;
    3. Southeast Gulf: Uniola; and
    4. Mid-Atlantic/Gulf: Spartina patens.
    D. Coastal tundra. This ecosystem, which is found along the Arctic 
and Boreal coasts of North America, is characterized by low 
temperatures, a short growing season, and some permafrost, producing a 
low, treeless mat community made up of mosses, lichens, heath, shrubs, 
grasses, sedges, rushes, and herbaceous and dwarf woody plants. Common 
species include arctic/alpine plants such as Empetrum nigrum and Betula 
nana, the lichens Cetraria and Cladonia, and herbaceous plants such as 
Potentilla tridentata and Rubus chamaemorus. Common species on the 
coastal beach ridges of the high arctic desert include Bryas 
intergrifolia and Saxifrage oppositifolia. This area can be divided into 
two main subdivisions:
    1. Low tundra: Characterized by a thick, spongy mat of living and 
undecayed vegetation, often with water and dotted with ponds when not 
frozen; and
    2. High Tundra: A bare area except for a scanty growth of lichens 
and grasses, with underlaying ice wedges forming raised polygonal areas.
    E. Coastal cliffs. This ecosystem is an important nesting site for 
many sea and shore birds. It consists of communities of herbaceous, 
graminoid, or low woody plants (shrubs, heath, etc.) on the top or along 
rocky faces exposed to salt spray. There is a diversity of plant species 
including mosses, lichens, liverworts, and ``higher'' plant 
representatives.

                       Group II--Transition Areas

    A. Coastal marshes. These are wetland areas dominated by grasses 
(Poacea), sedges (Cyperaceae), rushes (Juncaceae), cattails (Typhaceae), 
and other graminoid species and is subject to periodic flooding by 
either salt or freshwater. This ecosystem may be subdivided into: (a) 
Tidal, which is periodically flooded by either salt or brackish water; 
(b) nontidal (freshwater); or (c) tidal freshwater. These are essential 
habitats for many important estuarine species of fish and invertebrates 
as well as shorebirds and waterfowl and serve important roles in shore 
stabilization, flood control, water purification, and nutrient transport 
and storage.
    B. Coastal swamps. These are wet lowland areas that support mosses 
and shrubs together with large trees such as cypress or gum.
    C. Coastal mangroves. This ecosystem experiences regular flooding on 
either a daily, monthly, or seasonal basis, has low wave action, and is 
dominated by a variety of salt-tolerant trees, such as the red mangrove 
(Rhizophora mangle), black mangrove (Avicennia Nitida), and the white 
mangrove (Laguncularia racemosa.) It is also an important habitat for 
large populations of fish, invertebrates, and birds. This type of 
ecosystem can be found from central Florida to extreme south Texas to 
the islands of the Western Pacific.
    D. Intertidal beaches. This ecosystem has a distinct biota of 
microscopic animals, bacteria, and unicellular algae along with 
macroscopic crustaceans, mollusks, and worms with a detritus-based 
nutrient cycle. This area also includes the driftline communities found 
at high tide levels on the beach. The dominant organisms in this 
ecosystem include crustaceans such as the mole crab (Emerita), amphipods 
(Gammeridae), ghost crabs (Ocypode), and bivalve mollusks such

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as the coquina (Donax) and surf clams (Spisula and Mactra.)
    E. Intertidal mud and sand flats. These areas are composed of 
unconsolidated, high organic content sediments that function as a short-
term storage area for nutrients and organic carbons. Macrophytes are 
nearly absent in this ecosystem, although it may be heavily colonized by 
benthic diatoms, dinoflaggellates, filamintous blue-green and green 
algae, and chaemosynthetic purple sulfur bacteria. This system may 
support a considerable population of gastropods, bivalves, and 
polychaetes, and may serve as a feeding area for a variety of fish and 
wading birds. In sand, the dominant fauna include the wedge shell Donax, 
the scallop Pecten, tellin shells Tellina, the heart urchin 
Echinocardium, the lug worm Arenicola, sand dollar Dendraster, and the 
sea pansy Renilla. In mud, faunal dominants adapted to low oxygen levels 
include the terebellid Amphitrite, the boring clam Playdon, the deep sea 
scallop Placopecten, the Quahog Mercenaria, the echiurid worm Urechis, 
the mud snail Nassarius, and the sea cucumber Thyone.
    F. Intertidal algal beds. These are hard substrates along the marine 
edge that are dominated by macroscopic algae, usually thalloid, but also 
filamentous or unicellular in growth form. This also includes the rocky 
coast tidepools that fall within the intertidal zone. Dominant fauna of 
these areas are barnacles, mussels, periwinkles, anemones, and chitons. 
Three regions are apparent:
    1. Northern latitude rocky shores: It is in this region that the 
community structure is best developed. The dominant algal species 
include Chondrus at the low tide level, Fucus and Ascophylium at the 
mid-tidal level, and Laminaria and other kelplike algae just beyond the 
intertidal, although they can be exposed at extremely low tides or found 
in very deep tidepools.
    2. Southern latitudes: The communities in this region are reduced in 
comparison to those of the northern latitudes and possesses algae 
consisting mostly of single-celled or filamentour green, blue-green, and 
red algae, and small thalloid brown algae.
    3. Tropical and subtropical latitudes: The intertidal in this region 
is very reduced and contains numerous calcareous algae such as 
Porolithon and Lithothamnion, as well and green algae with calcareous 
particles such as Halimeda, and numerous other green, red, and brown 
algae.

                      Group III--Submerged Bottoms

    A. Subtidal hardbottoms. This system is characterized by a 
consolidated layer of solid rock or large pieces of rock (neither of 
biotic origin) and is found in association with geomorphological 
features such as submarine canyons and fjords and is usually covered 
with assemblages of sponges, sea fans, bivalves, hard corals, tunicates, 
and other attached organisms. A significant feature of estuaries in many 
parts of the world is the oyster reef, a type of subtidal hardbottom. 
Composed of assemblages of organisms (usually bivalves), it is usually 
found near an estuary's mouth in a zone of moderate wave action, salt 
content, and turbidity. If light levels are sufficient, a covering of 
microscopic and attached macroscopic algae, such as keep, may also be 
found.
    B. Subtidal softbottoms. Major characteristics of this ecosystem are 
an unconsolidated layer of fine particles of silt, sand, clay, and 
gravel, high hydrogen sulfide levels, and anaerobic conditions often 
existing below the surface. Macrophytes are either sparse or absent, 
although a layer of benthic microalgae may be present if light levels 
are sufficient. The faunal community is dominated by a diverse 
population of deposit feeders including polychaetes, bivalves, and 
burrowing crustaceans.
    C. Subtidal plants. This system is found in relatively shallow water 
(less than 8 to 10 meters) below mean low tide. It is an area of 
extremely high primary production that provides food and refuge for a 
diversity of faunal groups, especially juvenile and adult fish, and in 
some regions, manatees and sea turtles. Along the North Atlantic and 
Pacific coasts, the seagrass Zostera marina predominates. In the South 
Atlantic and Gulf coast areas, Thalassia and Diplanthera predominate. 
The grasses in both areas support a number of epiphytic organisms.

                   Class II--Physical Characteristics

                            Group I--Geologic

    A. Basin type. Coastal water basins occur in a variety of shapes, 
sizes, depths, and appearances. The eight basic types discussed below 
will cover most of the cases:
    1. Exposed coast: Solid rock formations or heavy sand deposits 
characterize exposed ocean shore fronts, which are subject to the full 
force of ocean storms. The sand beaches are very resilient, although the 
dunes lying just behind the beaches are fragile and easily damaged. The 
dunes serve as a sand storage area making them chief stabilizers of the 
ocean shorefront.
    2. Sheltered coast: Sand or coral barriers, built up by natural 
forces, provide sheltered areas inside a bar or reef where the ecosystem 
takes on many characteristics of confined waters-abundant marine 
grasses, shellfish, and juvenile fish. Water movement is reduced, with 
the consequent effects pollution being more severe in this area than in 
exposed coastal areas.
    3. Bay: Bays are larger confined bodies of water that are open to 
the sea and receive strong tidal flow. When stratification is pronounced 
the flushing action is augmented by

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river discharge. Bays vary in size and in type of shorefront.
    4. Embayment: A confined coastal water body with narrow, restricted 
inlets and with a significant freshwater inflow can be classified as an 
embayment. These areas have more restricted inlets than bays, are 
usually smaller and shallower, have low tidal action, and are subject to 
sedimentation.
    5. Tidal river: The lower reach of a coastal river is referred to as 
a tidal river. The coastal water segment extends from the sea or estuary 
into which the river discharges to a point as far upstream as there is 
significant salt content in the water, forming a salt front. A 
combination of tidal action and freshwater outflow makes tidal rivers 
well-flushed. The tidal river basin may be a simple channel or a complex 
of tributaries, small associated embayments, marshfronts, tidal flats, 
and a variety of others.
    6. Lagoon: Lagoons are confined coastal bodies of water with 
restricted inlets to the sea and without significant freshwater inflow. 
Water circulation is limited, resulting in a poorly flushed, relatively 
stagnant body of water. Sedimentation is rapid with a great potential 
for basin shoaling. Shores are often gently sloping and marshy.
    7. Perched coastal wetlands: Unique to Pacific islands, this wetland 
type found above sea level in volcanic crater remnants forms as a result 
of poor drainage characteristics of the crater rather than from 
sedimentation. Floral assemblages exhibit distinct zonation while the 
faunal constituents may include freshwater, brackish, and/or marine 
species. Example: Aunu's Island, American Samoa.
    8. Anchialine systems: These small coastal exposures of brackish 
water form in lava depressions or elevated fossil reefs have only a 
subsurface connection in the ocean, but show tidal fluctuations. 
Differing from true estuaries in having no surface continuity with 
streams or ocean, this system is characterized by a distinct biotic 
community dominated by benthis algae such as Rhizoclonium, the mineral 
encrusting Schiuzothrix, and the vascular plant Ruppia maritima. 
Characteristic fauna which exhibit a high degree of endemicity, include 
the mollusks Theosoxus neglectus and Tcariosus. Although found 
throughout the world, the high islands of the Pacific are the only areas 
within the U.S. where this system can be found.
    B. Basin structure. Estuary basins may result from the drowning of a 
river valley (coastal plains estuary), the drowning of a glacial valley 
(fjord), the occurrence of an offshore barrier (bar-bounded estuary), 
some tectonic process (tectonic estuary), or volcanic activity (volcanic 
estuary).
    1. Coastal plains estuary: Where a drowned valley consists mainly of 
a single channel, the form of the basin is fairly regular forming a 
simple coastal plains estuary. When a channel is flooded with numerous 
tributaries an irregular estuary results. Many estuaries of the eastern 
United States are of this type.
    2. Fjord: Estuaries that form in elongated steep headlands that 
alternate with deep U-shaped valleys resulting from glacial scouring are 
called fjords. They generally possess rocky floors or very thin veneers 
of sediment, with deposition generally being restricted to the head 
where the main river enters. Compared to total fjord volume river 
discharge is small. But many fjords have restricted tidal ranges at 
their mouths due to sills, or upreaching sections of the bottom which 
limit free movement of water, often making river flow large with respect 
to the tidal prism. The deepest portions are in the upstream reaches, 
where maximum depths can range from 800m to 1200m while sill depths 
usually range from 40m to 150m.
    3. Bar-bounded estuary: These result from the development of an 
offshore barrier such as a beach strand, a line of barrier islands, reef 
formations a line of moraine debris, or the subsiding remnants of a 
deltaic lobe. The basin is often partially exposed at low tide and is 
enclosed by a chain of offshore bars of barrier islands broken at 
intervals by inlets. These bars may be either deposited offshore or may 
be coastal dunes that have become isolated by recent seal level rises.
    4. Tectonic estuary: These are coastal indentures that have formed 
through tectonic processes such as slippage along a fault line (San 
Francisco Bay), folding or movement of the earth's bedrock often with a 
large inflow of freshwater.
    5. Volcanic estuary: These coastal bodies of open water, a result of 
volcanic processes are depressions or craters that have direct and/or 
subsurface connections with the ocean and may or may not have surface 
continuity with streams. These formations are unique to island areas of 
volcanic orgin.
    C. Inlet type. Inlets in various forms are an integral part of the 
estuarine environment as they regulate to a certain extent, the velocity 
and magnitude of tidal exchange, the degree of mixing, and volume of 
discharge to the sea.
    1. Unrestricted: An estuary with a wide unrestricted inlet typically 
has slow currents, no significant turbulence, and receives the full 
effect of ocean waves and local disturbances which serve to modify the 
shoreline. These estuaries are partially mixed, as the open mouth 
permits the incursion of marine waters to considerable distances 
upstream, depending on the tidal amplitude and stream gradient.
    2. Restricted: Restrictions of estuaries can exist in many forms: 
Bars, barrier islands, spits, sills, and more. Restricted inlets result 
in decreased circulation, more pronounced longitudinal and vertical 
salinity gradients, and more rapid sedimentation. However, if

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the estuary mouth is restricted by depositional features or land 
closures, the incoming tide may be held back until it suddenly breaks 
forth into the basin as a tidal wave, or bore. Such currents exert 
profound effects on the nature of the subtrate, turbidity, and biota of 
the estuary.
    3. Permanent: Permanent inlets are usually opposite the mouths of 
major rivers and permit river water to flow into the sea.
    4. Temporary (Intermittent): Temporary inlets are formed by storms 
and frequently shift position, depending on tidal flow, the depth of the 
sea, and sound waters, the frequency of storms, and the amount of 
littoral transport.
    D. Bottom composition. The bottom composition of estuaries attests 
to the vigorous, rapid, and complex sedimentation processes 
characteristic of most coastal regions with low relief. Sediments are 
derived through the hydrologic processes of erosion, transport, and 
deposition carried on by the sea and the stream.
    1. Sand: Near estuary mouths, where the predominating forces of the 
sea build spits or other depositional features, the shore and substrates 
of the estuary are sandy. The bottom sediments in this area are usually 
coarse, with a graduation toward finer particles in the head region and 
other zones of reduced flow, fine silty sands are deposited. Sand 
deposition occurs only in wider or deeper regions where velocity is 
reduced.
    2. Mud: At the base level of a stream near its mouth, the bottom is 
typically composed of loose muds, silts, and organic detritus as a 
result of erosion and transport from the upper stream reaches and 
organic decomposition. Just inside the estuary entrance, the bottom 
contains considerable quantities of sand and mud, which support a rich 
fauna. Mud flats, commonly built up in estuarine basins, are composed of 
loose, coarse, and fine mud and sand, often dividing the original 
channel.
    3. Rock: Rocks usually occur in areas where the stream runs rapidly 
over a steep gradient with its coarse materials being derived from the 
higher elevations where the stream slope is greater. The larger 
fragments are usually found in shallow areas near the stream mouth.
    4. Oyster shell: Throughout a major portion of the world, the oyster 
reef is one of the most significant features of estuaries, usually being 
found near the mouth of the estuary in a zone of moderate wave action, 
salt content, and turbidity. It is often a major factor in modifying 
estuarine current systems and sedimentation, and may occur as an 
elongated island or peninsula oriented across the main current, or may 
develop parallel to the direction of the current.

                         Group II--Hydrographic

    A. Circulation. Circulation patterns are the result of combined 
influences of freshwater inflow, tidal action, wind and oceanic forces, 
and serve many functions: Nutrient transport, plankton dispersal, 
ecosystem flushing, salinity control, water mixing, and more.
    1. Stratified: This is typical of estuaries with a strong freshwater 
influx and is commonly found in bays formed from ``drowned'' river 
valleys, fjords, and other deep basins. There is a net movement of 
freshwater outward at the top layer and saltwater at the bottom layer, 
resulting in a net outward transport of surface organisms and net inward 
transport of bottom organisms.
    2. Non-stratified: Estuaries of this type are found where water 
movement is sluggish and flushing rate is low, although there may be 
sufficient circulation to provide the basis for a high carrying 
capacity. This is common to shallow embayments and bays lacking a good 
supply of freshwater from land drainage.
    3. Lagoonal: An estuary of this type is characterized by low rates 
of water movement resulting from a lack of significant freshwater influx 
and a lack of strong tidal exchange because of the typically narrow 
inlet connecting the lagoon to the sea. Circulation whose major driving 
force is wind, is the major limiting factor in biological productivity 
within lagoons.
    B. Tides. This is the most important ecological factor in an estuary 
as it affects water exchange and its vertical range determines the 
extent of tidal flats which may be exposed and submerged with each tidal 
cycle. Tidal action against the volume of river water discharged into an 
estuary results in a complex system whose properties vary according to 
estuary structure as well as the magnitude of river flow and tidal 
range. Tides are usually described in terms of the cycle and their 
relative heights. In the United States, tide height is reckoned on the 
basis of average low tide, which is referred to as datum. The tides, 
although complex, fall into three main categories:
    1. Diurnal: This refers to a daily change in water level that can be 
observed along the shoreline. There is one high tide and one low tide 
per day.
    2. Semidiurnal: This refers to a twice daily rise and fall in water 
that can be observed along the shoreline.
    3. Wind/Storm tides: This refers to fluctuations in water elevation 
to wind and storm events, where influence of lunar tides is less.
    C. Freshwater. According to nearly all the definitions advanced, it 
is inherent that all estuaries need freshwater, which is drained from 
the land and measurably dilutes seawater to create a brackish condition. 
Freshwater enters an estuary as runoff from the

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land either from a surface and/or subsurface source.
    1. Surface water: This is water flowing over the ground in the form 
of streams. Local variation in runoff is dependent upon the nature of 
the soil (porosity and solubility), degree of surface slope, 
vegetational type and development, local climatic conditions, and volume 
and intensity of precipitation.
    2. Subsurface water: This refers to the precipitation that has been 
absorbed by the soil and stored below the surface. The distribution of 
subsurface water depends on local climate, topography, and the porosity 
and permeability of the underlying soils and rocks. There are two main 
subtypes of surface water:
    a. Vadose water: This is water in the soil above the water table. 
Its volume with respect to the soil is subject to considerable 
fluctuation.
    b. Groundwater: This is water contained in the rocks below the water 
table, is usually of more uniform volume than vadose water, and 
generally follows the topographic relief of the land being high hills 
and sloping into valleys.

                           Group III--Chemical

    A. Salinity. This reflects a complex mixture of salts, the most 
abundant being sodium chloride, and is a very critical factor in the 
distribution and maintenance of many estuarine organisms. Based on 
salinity, there are two basic estuarine types and eight different 
salinity zones (expressed in parts per thousand-ppt.)
    1. Positive estuary: This is an estuary in which the freshwater 
influx is sufficient to maintain mixing, resulting in a pattern of 
increasing salinity toward the estuary mouth. It is characterized by low 
oxygen concentration in the deeper waters and considerable organic 
content in bottom sediments.
    2. Negative estuary: This is found in particularly arid regions, 
where estuary evaporation may exceed freshwater inflow, resulting in 
increased salinity in the upper part of the basin, especially if the 
estuary mouth is restricted so that tidal flow is inhibited. These are 
typically very salty (hyperhaline), moderately oxygenated at depth, and 
possess bottom sediments that are poor in organic content.
    3. Salinity zones (expressed in ppt):
    a. Hyperhaline--greater than 40 ppt.
    b. Euhaline--40 ppt to 30 ppt.
    c. Mixhaline--30 ppt to 0.5 ppt.
    (1) Mixoeuhaline--greater than 30 ppt but less than the adjacent 
euhaline sea.
    (2) Polyhaline--30 ppt to 18 ppt.
    (3) Mesohaline--18 ppt to 5 ppt.
    (4) Oligohaline--5 ppt to 0.5 ppt.
    d. Limnetic: Less than 0.5 ppt.
    B. pH Regime: This is indicative of the mineral richness of 
estuarine waters and falls into three main categories:
    1. Acid: Waters with a pH of less than 5.5.
    2. Circumneutral: A condition where the pH ranges from 5.5 to 7.4.
    3. Alkaline: Waters with a pH greater than 7.4.